The invention relates to a microtome having a forward-feed control system for the specimen arm and/or the knife, and is especially applicable to an ultramicrotome.
In order to set the section thickness of the thin sections which are produced by means of a microtome, the specimen arm, carrying the preparation or specimen, is shifted towards the knife, before each cutting stroke, by an amount corresponding to the section thickness. In a known ultramicrotome, this is effected by means of a forward-feed mechanism. It is desirable to be able to set an ultramicrotome to produce section thicknesses which differ by several orders of magnitude so that, for example, when starting to cut up a preparation block, considerably thicker sections can initially be removed until a sectioning area, which can be used for a thin section proper, has itself been obtained on the preparation. In a known ultramicrotome, two preselector switches are provided for this purpose, the switches permitting substantially different section thicknesses to be set by the setting of forward-feed values. In an arrangement of this type, the forward-feed value desired for ultra-thin sectioning is set at one of the preselector switches while the forward-feed value desired during the initial cutting process is set at the other preselector switch. When the sectioning surface which produces the first usable thin section is reached on the preparation, the preselector switch, which defines the forward-feed amplitude during the initial cutting process, is turned to the stop position so that the ultramicrotome now operates with the forward feed which corresponds to the desired ultra-thin section thickness.
During the production of ultra-thin sections, it is frequently desired to produce a number of semi-thin sections, or it is necessary to "leap-frog" unusable parts of the preparation, for which purpose the forward feed is increased. In the case of the known ultramicrotome, this requires that the first preselector switch be reset. This proves to be troublesome, particularly during the deliberate inclusion of a few semi-thin sections, not only because resetting involves a delay, but also because the operator is distracted from the operations taking place on the preparation so that it is difficult to exclude the possibility of mistakes.